A Review on Synthesis and Physicochemical Properties-Photocatalytic Activity Relationships of Carbon Quantum Dots Graphitic Carbon Nitride in Reduction of Carbon Dioxide

Carbon dioxide (CO2) is a major greenhouse gas present in over half of the Earth's atmosphere. Elevated CO2 emissions in the atmosphere have become a global warming issue due to the excessive use of fossil fuels by human activities.  Converting CO2 into a useful compound is crucial since CO2 exists in the environment and must be reduced. The use of semiconductor materials in photocatalysis is the best solution to degrade and potentially convert CO2 into a useful energy source. Recently, research on graphitic carbon nitride (g-C3N4) has developed interest due to its phenomenal properties, such as effective charge separation and charge carrier lifetime, electron-hole recombination, and high surface area. CQDs/g-C3N4 has recently emerged as a novel technology due to its excellent physical and chemical properties, especially in the reduction of CO2. Thus, this review outlines the recent modification of graphitic carbon nitride (g-C3N4) by carbon quantum dots (CQDs), which include the synthesis of CQDs/g-C3N4 for CO2 reduction. Lastly, the review discusses physicochemical properties-photocatalytic activity relationship of CQDs/g-C3N4 in the photocatalytic reduction of CO2. This review provides a wide range of perspectives and a guideline for designing the more effective CQDs/ g-C3N4 for photocatalytic reduction of CO2.